1. Field of the Invention
The present invention relates to method and apparatus for analyzing a sample by irradiating the sample with an electron beam, spectrally dispersing and detecting the characteristic X-rays emanating from the sample in response to the irradiation by a wavelength-dispersive X-ray spectrometer, and measuring the intensities of the characteristic X-rays at the positions where characteristic X-ray peaks are detected.
2. Description of Related Art
In a sample analyzer having the structure of an electron probe microanalyzer (EPMA), a focused electron beam, or an electron probe, is directed at a sample, and characteristic X-rays emanating from the sample by the irradiation are spectrally dispersed and detected by a wavelength-dispersive X-ray spectrometer (WDS). The intensities of characteristic X-rays at the positions where characteristic X-ray peaks are detected are measured. Based on the results of the measurements of the characteristic X-ray intensities, qualitative analysis can be performed at the analysis position on the sample.
One example of peak profile of characteristic X-rays spectrally dispersed by a wavelength-dispersive X-ray spectrometer is shown in FIG. 1, where a characteristic X-ray peak A is shown. The peak intensity detected at the characteristic X-ray peak A is Ip. The peak intensity Ip includes background intensity Ib at the position of the peak A. A value In obtained by subtracting the background intensity Ib from the peak intensity Ip is the net characteristic X-ray intensity at the peak A.
Accordingly, when quantitative analysis of a sample is made using characteristic X-rays spectrally dispersed by a wavelength-dispersive X-ray spectrometer, the following operations need to be performed. The background intensity at the position where a characteristic X-ray peak is detected is found. The background intensity is subtracted from the detected peak intensity, thus calculating the net characteristic X-ray intensity. Quantitative analysis is made based on the characteristic X-ray intensity.
In this case, it is impossible in principle to directly detect only the background intensity Ib at the characteristic X-ray peak A. Therefore, the average value of the background intensity BL at a background position on the shorter wavelength side of the position of the peak A and the background intensity BH at a background position on the longer wavelength side is calculated. The net characteristic X-ray intensity In is found based on the above-described method while taking the average value as the background intensity Ib at the peak position.
Another conceivable method disclosed in Japanese Patent Laid-Open No. S63-313043 consists of estimating background by finding the relationship between the mean atomic number and the background from plural reference samples, measuring only peak intensities during measurement of unknown samples, and finding the mean atomic number from the measured values during calculations for quantitative corrections.
A sample analyzer can consist of an electron probe microanalyzer equipped with a wavelength-dispersive X-ray spectrometer (WDS) and also with an energy-dispersive X-ray analyzer (EDS). In a method being discussed (see Japanese Patent Laid-Open No. S59-214743), each value counted by the EDS is multiplied by a constant coefficient. The resulting product is used as the background component of the characteristic X-ray peak derived by the WDS.
In order to measure the peak intensities of characteristic X-rays spectrally dispersed by a wavelength-dispersive X-ray spectrometer and to measure background intensities at background positions respectively on the shorter and longer wavelength sides of the peak detection position, it is necessary to move an analyzing crystal into the background positions on the shorter wavelength side and on the longer wavelength side, respectively. Consequently, there is the problem that the time taken to move the analyzing crystal prolongs the measurement time.
Especially, when measurements are made at multiple analysis positions within a given region on a sample, it is necessary to move the analyzing crystal as described above into each analysis position. Therefore, a much longer measurement time is required as compared with the case in which only peak intensities are measured.
A sample analyzer equipped with a wavelength-dispersive X-ray spectrometer and an energy-dispersive X-ray spectrometer suffers from a similar problem. The analyzer described in the above-cited Japanese Patent Laid-Open No. S59-214743 is designed to solve this problem.